A therapeutic system comprising an ultrasound therapy unit and a MR imaging unit is generally known, e.g., from WO 2008/152542 A2.
Ultrasound is becoming an increasingly desirable approach for specific therapeutic interventions. In particular, the use of high intensity focused ultrasound is currently being used as an approach for thermal therapeutic intervention for uterine fibroids and has been examined for possible uses in the treatment of liver, brain, prostate, and other cancerous lesions. Ultrasound has also been the subject of much research as a means for mediating clot dissolution (sonothrombolysis), and has been shown to increase the efficacy of existing medical treatments such as the use of tissue plasminogen activator (tPA) as a thrombolytic agent for stroke patients. Ultrasound mediated drug delivery and gene therapy is a further active area of research. Genetic expression of proteins in gene therapy and increased delivery of drugs in site-targeted therapies have potential to treat a wide variety of diseases with minimal side-effects. Another application for ultrasound therapy is non-invasive treatment for cosmetic means, e.g., removal of fat. The use of ultrasound in all of these applications is desirable because it allows the non-invasive treatment of deep tissues with little or no effect on overlying organs.
Ultrasound therapy for tissue ablation works by insonifying a tissue of interest with high intensity ultrasound that is absorbed and converted into heat, thereby raising the temperature of the respective tissues. As the temperature rises above 55 degree centigrade, coagulative necrosis of the tissues occurs resulting in immediate cell death. The transducers used in therapy can be outside the body or be inserted into the body e.g. through blood vessels, urethra, rectum etc. However, ultrasound therapy is not limited to tissue ablation, but also relates to the use of other types of ultrasound-based bio-effects, including hemostasis, drug or gene delivery, clot dissolution etc.
Nowadays, MR imaging guided high intensity focused ultrasound (MR HIFU) systems are commercially available. The first clinical application is the ablation of benign tumours in the uterus, so-called intrauterine fibroids. Therein a focused ultrasound beam is directed towards the abdomen. The ultrasound beam is used for heating a tumour through the skin and intervening tissue while MR imaging is used for monitoring the temperature distribution within the insonified region. The latter makes the procedure safe and efficient.
MR thermometry, based on the proton resonance frequency shift (PRFS) in water, is considered the ‘gold standard’ in the non-invasive monitoring of such ablative thermal therapies. Using the PRFS method the temperature in tissues having a high water content can be monitored accurately. A linear shift of the proton resonance frequency is observed for the range of temperatures being used in HIFU. In this range MR thermometry is also reasonably sensitive. The reconstruction of thermographic MR images during ultrasound therapy is useful to provide feedback to ensure that adequate heating is accomplished at the intended location while safeguarding that other critical anatomic structures are left intact.
A drawback of MR thermometry is that tissues containing fat cannot be monitored. This is because the PRFS in fat is essentially independent of temperature. An example of this is the subcutaneous fat layer. For example, patients being treated for intrauterine fibroids usually have a fat layer under the skin of up to a few centimeters. The focused ultrasound beam used to ablate deep seated tissues has to pass through this layer. The fat, having both lower heat conductivity and no vascularity, is easily overheated. For many patients this becomes a risk, limiting the usefulness and applicability of the MR HIFU method.
From the foregoing it is readily appreciated that there is a need for an improved therapeutic system for MR imaging guided HIFU. It is consequently an object of the invention to enable the detection of unintended sites of possible thermal damage, thereby improving the safety of the therapeutic procedure.